1. Technical Field
This disclosure is related to the field of determining the dielectric properties of materials. In relation to measurements of subsurface rock formation properties, the disclosure pertains to dielectric measurements of such formations and dielectric well log interpretation.
2. Background Information
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the subject matter described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, not as admissions of prior art.
Formation water content in porous subsurface formations is one of the petrophysical quantities provided by dielectric measurements (e.g., obtained by well logging) of such formations. However, to determine formation water content from the formation dielectric measurements, the formation rock matrix permittivity must be known. Uncertainty in the rock matrix permittivity values translates into uncertainty in the formation water content estimate, which is especially important in low-porosity formations or complex lithologies (rock matrix mineral compositions). Generally, the matrix permittivity values are not well known for a number of formation minerals and can also vary for the same type of mineral if it forms part of different formations.
An approach for measuring permittivity of solid powder phase based on “no-contrast conditions” has been proposed that can also be applied to rock formation materials. See Robinson, D. A., 2004, “Measurement of the solid dielectric permittivity of clay minerals and granular samples using a TDR immersion method,” Vadose Zone Journal, 3 (2), pp. 705-713. The approach described in the foregoing publication includes a series of measurements on a powder filled with variable permittivity liquids. When the measured permittivity of the powder/liquid mixture equals the permittivity of the saturating liquid, so-called “no contrast” conditions are met. Under such no contrast conditions, the permittivity of the solid powder phase equals the permittivity of the saturating fluid. Published methodologies required a demanding measurement technique that had to be practiced before reliable data could be obtained, which may make such methods impractical.
Another implementation of the no-contrast methodology involving measurements on liquid/powder mixtures was based on measurements made using a capacitive measurement cell. Capacitive measurement cells may make inferior accuracy measurements compared with other available techniques and may also be limited in measurement frequency span. Such limitations may not allow the measurement to be easily extended to 1 gigahertz (GHz) range, which is important in dielectric well logging for determination of water-filled porosity. This is because some dielectric well logging instruments make measurements in the 1 GHz frequency range.
Other attempts to study dielectric properties of dry rocks included utilization of a dielectric probe. Use of such probes is a fast and convenient measurement technique, but also has lesser accuracy as compared to some other techniques. Generally, it requires good contact between the sample and the probe surfaces, and is only sensitive to a part of the sample volume which is adjacent to the probe. See Ulaby, T. F., Bengal, T. H., Dobson, C. M., East, J. R., Garvin, J. B., and Evans, D. L., 1990, “Microwave Dielectric Properties of Dry Rocks”, IEEE Trans. On Geoscience and Remote Sensing, vol. 28, No. 3, 325-336.